The present invention relates to junction field effect transistors.
Junction field effect transistor (JFET) devices are used as charge amplifiers because they may be designed with a very high input impedance. Charge amplifiers are required to amplify the output from transducers of the type which produce small charge or current signals. Transducers of this type include, for example, those sensitive to ionising radiation such as gamma rays and x-trays. In x-ray fluorescence equipment, for example, x-rays (generated by an x-ray tube or as a result of electron bombardment in an electron microscope) are analysed to determine the elemental composition of a specimen. Equipment of this type is disclosed in the Applicants co-pending UK Patent Application No. 8718531 (LA2).
Usually an FET is used as the first stage of a charge amplifier which feeds other amplifier stages to produce an amplified charge signal which is in turn supplied to a processing unit. The processing unit is arranged to measure the charge signal to provide information relating to the conditions experienced by the transducer. Given that it is usual for the charge signal to change increase say, in only one direction the accumulated charge must be removed before the system starts to saturate and become non-linear; a process commonly known as restore.
A problem with restoring charge amplifiers, as described above, is that the amount of accumulated charge is often very small thus the required restore current may be a few nano amps required for less than ten micro seconds. The existance of stray capacitance in any electronic system makes direct control of a current this size virtually impossible over such a small time scale.
A known solution to the problem removing charge from amplifiers of the aforesaid type is that of "pulsed optical restore" as described in UK Patent No. 1153374. In this system an accumulated charge is discharged by a light responsive semiconductor device which in turn receives light from a light emitting semiconductor device. A short pulse of relatively high current is directly applied to the light emitting device, however, the current generated by the light responsive device (positioned as close as possible to the accumulated) charge to minimise the effect of stray capacitance, is several orders of magnitude smaller than the first current.
A problem with pulsed optical restore is that, in many applications (including x-ray fluorescence), light from the light emitting device must not reach the input transducer, x-ray detector say. However shielding placed between the transducer and the light emitting device introduces capacitance. This problem is partially solved by arranging the light shield to be part of a feedback capacitance, however, opaque materials suitable for this application do not have good dielectric properties and a introduce noise.
A further problem with pulsed optical restore is that the light sensitive device introduces another stray capacitance. This problem may be overcome by designing the FET itself to be sensitive to light and then directing the restore light onto the FET. However such an approach causes charge to build up on the FET and additional time may be required to remove this charge before measurements may be taken.